Non-prescription antibiotic use is an understudied problem, prevalent in some low-income communities. This practice includes obtaining and taking antibiotics without a prescription (e.g., from ethnic stores or flea markets), taking another person's antibiotics, or taking one's own stored antibiotics for an indication other than that for which the antibiotic was originally prescribed. In our survey of low-income primary care patients in a large urban area, respondents from public clinics were more likely than respondents from private clinics to report intention to use antibiotics without a prescription. This survey generated considerable press interest, as the problem has barely been described in the United States, but we did not explore why the non-prescription use was occurring. Non-prescription use increases the risk of development of antibiotic resistance, adverse drug reactions, damage to the microbiome, and other harms. There is a critical need to identify the predictors of non-prescription use that will guide development of effective antimicrobial stewardship interventions in low- income populations, given that these populations are at higher risk of developing antibiotic-resistant infections. The overall objectives of the proposed project are to identify the predictors of non-prescription antibiotic use in diverse, predominantly uninsured patients of safety net clinics and lay the foundation for a subsequent intervention. We will use mixed methods and the Kilbourne et al. (2006) conceptual framework for advancing health disparities research to identify the predictors of non-prescription use. In Aim 1 (quantitative phase), we will administer a survey in Spanish and English to assess how patient (e.g., health literacy), health care system (e.g., access to care), and clinical encounter factors (patient-provider communication) are associated with non- prescription antibiotic use in low-income patients. In Aim 2 (qualitative phase), using in-depth interviews, we will explore patient perspectives and experiences with non-prescription use among a racially/ethnically diverse subsample of survey respondents who reported using antibiotics without a prescription. In Aim 3 (development of a communication tool), we will integrate quantitative and qualitative results into a communication tool to be used in the clinics to steer patients toward safer antibiotic use and healthcare options. Our team includes an epidemiologist, an infectious diseases physician, a health disparities researcher, health communications experts, and a medical anthropologist. This multidisciplinary team will work with a Community Advisory Board comprised of patient representatives from participating clinics. Our work will be guided by input from a representative from the Centers for Disease Control and Prevention and two health literacy consultants, one of whom is bilingual. The proposed research is innovative, because it will provide comprehensive understanding of the predictors of this unsafe use of antibiotics in a low-income population. The research is significant, because it will inform understanding of disparities in use of antibiotics and future development of interventions to reduce harms associated with non-prescription use, such as the spread of antibiotic-resistant bacteria.